The present invention relates to a servo controller and an optical disk device to record or to reproduce information on an optical disk.
In an optical disk device employing an optical disk, periodic external disturbance due to rotation of the optical disk is a factor to cause a servo following capability. Methods to improve the following performance to follow such periodic disturbance are described, for example, in JP-A-8-77589 and JP-A-2007-207390.
According to JP-A-8-77589, for example, in the description of paragraph [0056], track deviation obtained for each of several preceding disk rotations is stored to be compared with the present track deviation. As a result of the comparison, a correlation is detected between the tracking error of the track currently being followed and a mean value of tracking errors for the several tracks. Based on the correlation, quantity of attenuation is changed in the feedforward loop of a learning compensation section and the loop gain of a tracking control system is changed, to thereby change the degree of learning and the control band according to the track correlation.
As periodic disturbance components which appear during rotation of an optical disk, a component “deviation” is known in addition to an eccentricity component and a plane fluctuation component. According to JP-A-2007-207390, in an optical disk including a guide groove, a groove-shaped defective section exists due to deterioration of a disk stumper and/or disk forming defect depending on cases. When the disk rotates at a high operation speed, a signal reflected from the groove contains a noise component inherent to a broad band particularly in an outer circumferential section of the disk. JP-A-2007-207390 also describes a technique to detect a deviation of the guide groove of the optical disk by use of quantity of variation in an obtained focusing or tracking error signal.
In the servo error signal employed in the control operation of the focusing system and/or the tracking system, a component which the servo system cannot suppress appears as a residual error. Hence, when an optical disk has the plane fluctuation and/or the eccentricity, the servo error signal contains a signal variation component of the rotary period. Similarly, when a track on an optical disk has a partial distortion (to be referred to as a deviation hereinbelow), the signal variation component is observed as a high-frequency signal variation in the servo error signal.
FIG. 35 graphically shows a servo error signal when a deviation is passed. Vref indicates a reference level of the servo error signal. A high-frequency signal variation component observed in the servo error signal when a deviation is passed will be referred to as a deviation component hereinbelow. The deviation component corresponds to waveforms A in the graph of FIG. 35. Trot represents a rotary period, and the servo error signal contains a component having the period Trot. This component is an eccentric component in the tracking servo system and a plane fluctuation component in the focusing servo system.
As FIG. 35 shows, the deviation component takes place at timing synchronized with the rotary period. This results in substantially an equal signal variation waveform at an interval of the rotary period. The signal deviation is a local distortion due to a production process of the disk and hence appears at a particular angle during the disk rotation. Additionally, a track adjacent to the track under consideration is also distorted in almost the same contour in the focusing direction and the tracking direction.
The deviation is due to defect in the track shape on a recording surface of an optical disk and hence particularly exists in a local range of its radius on the recording surface.
FIG. 36A schematically shows a servo error signal when a deviation is passed. In this graph, the abscissa represents time. On the optical disk, tracks are formed in a direction from an inner circumference to an outer circumference. Hence, the abscissa of FIG. 36A may also be regarded as a radial position r of a track currently being followed, relative to a center of the optical disk. FIGS. 36A and 36B show a situation in which a deviation exists in an area ranging from radius r1 to radius r2.
The present inventor has detected characteristics of deviation component described below. The deviation component appears at a first radial position (r1 in FIG. 36A) and then becomes gradually greater in amplitude. And the amplitude becomes gradually smaller toward a second radial position (r2 in FIG. 36A) and no deviation component exists in an area of a radius more than r2. The range of radius r (r1≦r≦r2) in which the deviation exists will be referred to as a deviation area hereinbelow.
The deviation component is lower in frequency than disturbance components such as those of a defect and a scratch. Hence, for an effective following operation, it is efficient to suppress the deviation component by increasing the servo gain. On the other hand, the disturbance components such as those of a defect and a scratch have a high frequency. Hence, it is efficient to reduce the servo response by lowering the servo gain not to forcibly conduct the following operation.
The deviation, eccentric, and plane fluctuation components have signal waveforms synchronized with the rotary period and can be represented through Fourier transform as a sum of a rotary period component and its higher-order components. Therefore, by suppressing these components through iterative learning control, it is possible to improve performance of the following operation when a deviation is passed.
For the tracks on an optical disk recording surface as objects of the following operation of the optical disk device, the optical disk standards prescribe physical precision in the focusing and tracking directions. For an optical disk conforming to the standards, it is guaranteed that the components described above can be suppressed by using a servo characteristic prescribed by the standards. However, optical disks having a large deviation, i.e., optical disks other than those conforming to the standards may actually be put to the market. To appropriately cope with such optical disks, the optical disk device needs to have servo performance of the following operation which is higher in efficiency than that prescribed by the standards. On the other hand, there also exists a need in which the period of time for the recording and reproducing operations is reduced by coping with the recording and reproducing operations at a high operation speed equal to or more than the operation speed prescribed by the standards for an optical disk conforming to predetermined standards. In this situation, the following performance which is higher than that assumed by the standards is also required.
In the situation in which the following performance higher than that assumed by the standards is required as the servo performance of the optical disk device, there occurs an event in which the suppression gain is particularly insufficient for the deviation component. In a worst case, the following operation cannot be appropriately carried out and the servo operation fails. Even if the servo operation does not fail, since the following error becomes larger when a deviation is passed, the recording and reproducing performance is deteriorated. Therefore, it is required to improve the following performance for the deviation.
To remove the problem, it is required to change the servo characteristic to improve the suppression performance for the deviation component. However, in general, when the suppression performance is improved for a particular frequency range, the following performance is disadvantageously deteriorated for other frequency ranges.
To improve the suppression performance for the deviation component, the gain of the servo characteristic is uniformly increased, for example, as shown in FIG. 37. However, in this case, the gain margin lowers in the frequency indicated by A in FIG. 37. Hence, the following performance is deteriorated for the disturbance component higher in the frequency than the servo band. Hence, it is feared that the following performance is deteriorated when a defect or a scratch is passed.
The inventor has recognized a fact as below. It is favorable in consideration of the aspect of the deviation shown in FIG. 36A that by detecting a local radial area (deviation area) in which a deviation is present, the servo characteristic is changed only in the deviation area.
For the operation, it is first required to detect the starting point of the deviation area, and then the servo characteristic is changed to increase the degree of suppression for the deviation component. In a state in which a track is being followed by use of a servo characteristic with the increased degree of suppression for the deviation component, it is required that when an end edge of the deviation area is passed, the end edge of the deviation area is detected to restore the servo characteristic to a steady-state characteristic.
However, in the prior art, the deviation is detected on the basis of the servo error signal. Hence, although the first point of the deviation area can be detected, it is not possible to detect the end point of the deviation area.
For example, JP-A-2007-207390 describes a configuration in which the deviation is detected according to the focusing or tracking error signal. When the deviation is detected on the basis of the servo error signal to improve the suppression performance for the deviation component as in the detection method described above, a problem takes place. The problem will be described by referring to the waveforms shown in FIGS. 36A and 36B.
FIG. 36A shows a servo error signal in an operation to follow a track in a deviation area by use of a steady-state characteristic. When the steady-state characteristic is used, the deviation component of the serve error signal has large amplitude in the deviation area. Hence, it is possible to easily detect the deviation in the method described above. Therefore, as FIGS. 36A and 36B show, when a deviation area is passed during the track following operation, although the deviation area cannot be immediately detected at the first point thereof since the amplitude of the deviation component is small, the deviation can be detected when the amplitude becomes greater, for example, a value corresponding to the radius (rd in FIG. 36A) exceeding a predetermined voltage level (Vt in FIG. 36A). Hence, the first point of the deviation area can be detected for radius rd.
FIG. 36B shows a servo error signal in an operation to follow a track in the same deviation area as that of FIG. 36A. In the operation, the first position of the deviation area is detected at radius rd and a servo characteristic resultant from the uniform increase in the servo gain is employed in a range represented as r≧rd.
By uniformly increasing the gain of the servo characteristic as above, the deviation component is suppressed to lower the track following error. However, the deviation component is reduced in the servo error signal. In this situation, whether or not a deviation is present in the track being currently followed cannot be determined. That is, since the deviation component is too small, the area actually including the deviation cannot be detected in the method described above.
This leads to a problem in which even the end edge of the deviation area is passed, the event of the passing of the end edge cannot be detected. It is difficult to restore to the steady-state characteristic in response to the end edge of the deviation area because of the increased servo gain.
By dividing the surface of an optical disk into predetermined areas according to the radius of the disk, it is possible to restore the servo characteristic to the steady-state characteristic at timing when the operation moves from a first area to a second area. However, this method is not available if the deviation area exists in two adjacent areas over a boundary between the two areas. In this situation, if the servo characteristic is restored to the steady-state characteristic at timing at which the operation moves from the first area to the second area, the servo control fails in a worst case when the deviation is passed. In a recording operation, this results in recording failure.
As a method to improve the suppression characteristic for the deviation component, a method to uniformly increase the gain of the servo characteristic has been described. Next, description will be given of a method employing iterative learning control.
For this purpose, a method to change the learning degree of the iterative learning control is known. As described in JP-A-2007-207390, the learning degree of the iterative learning control can be set, for example, by changing the value (to be represented by K hereinbelow) of the variable gain of the iterative learning control system. For the depression of the deviation as an object of the present invention, it is only required to increase the value K of the variable gain of the iterative learning control system in the deviation area.
However, also in the case in which the deviation is suppressed by increasing the learning degree of the iterative learning control, the deviation component is suppressed and the following error is reduced, but the deviation component becomes smaller in the servo error signal as in the case shown in FIG. 36B. Therefore, it is similarly difficult to restore the characteristic from the characteristic in which the servo gain is increased after detection of the end edge of the deviation area to the steady-state characteristic.
According to the prior art, it is not clarified whether or not a condition (such as a threshold value) to detect the deviation is changed, when the variable gain is changed, before and after the variable gain change. Moreover, the signal and the process to be employed to correctly detect the deviation area when the variable gain is changed are not clarified.
Hence, in the prior art, there exists a problem in which it is not possible to change the servo characteristic only in the deviation area by detecting a deviation area.